How to ground a Grounding Autotransformer

[newschool]

Hello all this is in reference to the 2008 NEC 450.5

"Zig-zag connected transformers shall not be installed on the load side of any system grounding connection..."

The service is a 480V 3ph/4W and we are extending a 3ph/3w feeder to a photovoltaic inverter. At the inverter we are providing a grounding transformer in the form of a zig-zag autotransformer to comply with local utility requirements.

So I understand that we are not to connect to the load side of any system grounding but can I assume that running a 3ph/3w feeder doesn't apply here? If this is the case then do we create a new system ground at the grounding transformer and tie to a new grounding electrode system? I have read else where that the neutral point floats but then what do we do with the equipment grounding?

Thanks in advance.

 

[winnie]

The whole point of a zig-zag transformer is to create a low impedance neutral point given a balanced three phase supply. If you supply a zig-zag transformer from a _grounded_ system, and then ground the derived neutral point, then you are essentially 're-grounding' your neutral, and setting up conditions where you will be injecting current into your equipment grounding conductors.

I don't know, if for your purposes, you can use a zig-zag transformer to create a neutral that doesn't get locally grounded. When supplied by a grounded system, the derived neutral will be at approximately ground potential (similar to the voltage of a hard wired neutral with current and voltage drop in the wires).

If you require a locally grounded neutral at the end of a 3 wire feeder, then you will need a normal transformer with separate primary and secondary, not an autotransformer.

-Jon

 

[kwired]

Hello all this is in reference to the 2008 NEC 450.5

"Zig-zag connected transformers shall not be installed on the load side of any system grounding connection..."

The service is a 480V 3ph/4W and we are extending a 3ph/3w feeder to a photovoltaic inverter. At the inverter we are providing a grounding transformer in the form of a zig-zag autotransformer to comply with local utility requirements.

So I understand that we are not to connect to the load side of any system grounding but can I assume that running a 3ph/3w feeder doesn't apply here? If this is the case then do we create a new system ground at the grounding transformer and tie to a new grounding electrode system? I have read else where that the neutral point floats but then what do we do with the equipment grounding?

Thanks in advance.

Outside of code requirements you only have three choices, ground your neutral, ground another system conductor, utilize it ungrounded.

An autotransformer can only have one point grounded, grounding multiple points will result in fault current flowing between those two points.

A separately derived secondary can have any single point grounded regardless of what is (or isn't) grounded in the supply system. Code may dictate which conductor is supposed to be grounded though.